One technique for providing soft, uniform lighting is to direct light from a source into a hollow light pipe having walls which selectively resist the passage of light. Since only a fraction of the light stricking a wall is able to escape, the pipe disperses the light along its length.
An example of a wall material for use in such light pipes is an acrylic plastic sheet marketed by the Minnesota Mining and Manufacturing Co. of St. Paul, Minn. under the trademark SOLF. The sheet, approximately 0.022 inch in thickness, has a smooth inner surface and a grooved outer surface forming a series of 90.degree. isosceles prisms running along the length of the sheet. Light entering the sheet through the inner surface at an angle less than or equal to 27.6.degree. from parallel to the sheet is trapped and reflected back through the inner surface, while light striking the sheet at a steeper angle is transmitted through the sheet. Since a hollow pipe composed from SOLF is able to distribute and conduct light along its length, the tube is able to distribute the intensity of exiting light across its length.
The distribution of light which exits the light pipe may be controlled to some degree by inserting an "extractor," i.e. a sheet of reflective material, into the light pipe. The "extractor" acts as a light releasing mechanism to turn a portion of the light at an angle steep enough to exit.
A reflector is usually provided to capture light emanating from the source and directing the light into the pipe. In one device, a lamp is placed at a focus of either a parabolic or elliptical reflector which opens directly into the light pipe. Light emitted from the lamp away from the light pipe is collected by the reflector and directed back into the pipe.
While inexpensive, this device has drawbacks. One drawback of parabolic and elliptical reflectors is size. Another drawback is they do not direct light uniformly over the cross-section of the pipe entrance. Since the lamp is directly exposed to the pipe, the intensity of light entering the pipe is heavier near the center than toward the sides. As a result, the intensity of the light leaving the pipe is less over its surface area than if the distribution of light across the entrance of the pipe were uniform. This problem is all the more significant if the reflector is used with one of the smaller, higher intensity bulbs recently developed in order to conserve energy.
Furthermore, the light pipe is exposed to heat from the lamp which could damage the pipe or transfer unwanted heat into the area being illuminated.
Another drawback of the device is that light is reflected back into the source, resulting in a waste of light.
Various reflector shapes have been proposed for other applications which, had they ever been altered or adapted for coupling to a light pipe, might isolate the light source from the reflector outlet area. These reflectors are often formed in one piece and have a continuous profile around the source. These profiles may be composed of a single curve, such as an ellipse, circular involute or circular spiral, or may be built up from segments of several curves. Since the outlet is at least partially shielded from the direct glare of the lamp, the "bright spot" which might otherwise appear at the exit is attenuated.
For example, the "paracyl" reflector is comprised of a section with a circular profile and a section with a parabolic profile. The profile is "continuous" at the point where the sections meet, i.e. the profile curves have the same slope. The lamp is partially isolated within the circular section and the light is directed out of the reflector by the parabolic section.
While reflectors such as the paracyl help isolate the light source from the exit, they are not very efficient in directing light to their outlets with the degree of alignment which would facilitate conduction of the light were a pipe coupled to their outlet. Indeed, light emitted from the lamp at certain angles is either reflected back into the source or lost in the reflector so that more power is required to maintain the same intensity of illumination.